How to identify canker sores.

- Characteristics. / Stages of formation. / Pictures.

Canker sore

Link to ulceration diagram.

Canker sore

Link to ulceration locations animation.

Dentists distinguish canker sores (also known as recurrent minor aphthous ulcers, the type of canker lesion that affects 80% of sufferers) from other types of mouth sores by way of:

  1. Their appearance.
  2. Their location and the type of tissue on which they've formed.
  3. The fact that they are recurrent (have formed repeated).

The dentist will also quiz their patient about the signs and symptoms that they experienced (or didn't experience) both immediately preceding and during their ulcer's formation.

There's usually no medical testing (biopsy or culturing) needed to make a diagnosis.

What do they look like?

Stage 1 - (24 hours duration)

The earliest sign of canker sore development usually isn't visible. It involves just the sensation of tingling, burning or numbness in the area (see below) where the lesion will ultimately form.

A slideshow of pictures of aphthous ulcers.

Pictures of canker sores (aphthous ulcers).

Stage 2 - (18 to 72 hours duration)

The earliest visible signs have now started to form. This includes the formation of a circular raised, reddened area on the skin. The lesion gradually develops a white membrane coating.

Stage 3 - (1 to 14 days duration)

At this point, the membrane sheds off revealing a well defined ulcer as describe below. The sore may continue to grow in size for 4 to 6 days.


The physical characteristics of a canker sore.

A diagram showing the physical characteristics of an aphthous ulcer.

Sores have a grayish membrane-coated central ulceration with a surrounding red border. The skin around the lesion looks normal.


  • A single shallow ulcer having a symmetrical round or oval shape. (Multiple lesions are possible, see below.)
  • Sores are typically no more than 1/4 inch in diameter.
  • The lesion's center will have a loosely attached yellow or grayish membrane.
  • Its border will be circumscribed (well defined) and regular (not jagged), and surrounded by an erythematous (reddish) halo.
  • The skin beyond the reddened border will appear normal and healthy.

Other signs and symptoms.

  • There usually is some level of pain associated with the lesion. People experiencing a sore will frequently limit their oral activities so not to disturb it.
  • There are no distinguishing systemic features such as the person having fever or malaise (feeling poorly).

Where do canker sores form?

They usually only form on the "loose" (movable) tissues of the mouth, meaning those areas where the skin is not tightly bound to the bone underneath. (These are also the "nonkeratinized" tissues of the mouth.)

Where canker sores usually form.

Animated graphic showing the mouth locations where aphthous ulcers typically form.

The ulcerations form on the "loose" tissues of the mouth.

Areas that have this type of tissue covering are the:

  • Inside of the lips and cheeks.
  • The floor of the mouth.
  • The tip or underside of the tongue.
  • The soft palate.
  • The tonsillar areas.

[ While rare, it's possible for canker sores to form on the topside of the tongue or on keratinized tissues, such as the gum tissue that surrounds a person's teeth. ]

Healing time frames.

  • A canker sore will usually heal within 4 to 14 days, although some can heal in as little as 3 to 5. In most cases this healing is uneventful and results in no residual scarring. (Woo 1996) [page references]
  • New skin growth over the healing ulcer will start at its edges, and then continue to close in from all directions.

    The gradual progress of this process will ultimately cover over the wound. As this takes place, the loose white/gray membrane that was originally present will ultimately be replaced by new pink skin.

    The reddened border of the original wound will remain obvious during the early stages of the healing process. Then, as the new skin tissue matures, the border's redness will fade. Complete healing and a normal appearance will return to the affected area within the time frame stated above.

  • The pain associated with the lesion usually starts to subside around day 3 to 4.
  • Any ulcer that has not healed within a 2 week time frame should be evaluated by a dentist.
  • There is another, larger, kind of aphthous ulcer (see below) that characteristically does have an extended healing time frame.

Outbreak frequency.

  • Once a person has experienced an initial outbreak the probability of recurrence is high, although the frequency of formation can be quite variable.
  • A rate of one outbreak every 1 to 3 months could be considered typical (encompassing 50% of people who get them). 30% of sufferers deal with their presence on a monthly basis.
  • It's been estimated that roughly 20% of the general population experiences canker sores.
  • A person's first outbreak will typically take place between the ages of 10 and 20 years and then decrease in frequency and severity with age. Lesions are most prevalent in people ages 10 to 40.
  • It's been suggested that women are more likely to experience sores than men but not all studies confirm this fact.
  • Family predisposition seems to exist. Children with parents who get canker sores have a 90% chance of developing them as compared to 20% for those whose parents don't.
  • A person's risk seems to be unrelated to race. But people who live in North America are more likely to suffer from them, as opposed to those who live on other continents.
  • Non-smokers and people in upper socioeconomic groups are more likely to experience outbreaks.

(Chavan 2012, Ship 1972, Ship 2000, Woo 1996)

Can a person have more than one at a time?

  • Outbreaks of multiple canker sores can and do occur (in contrast to the classic single-ulcer form).
  • Usually the total number of sores that will form at one time will be six or fewer. If multiple canker sores do develop they tend to be widely distributed throughout the person's mouth (as opposed to being clustered together).
  • If two sores form close to each other, they may combine into a single larger and irregularly shaped ulcer (as opposed to the classic round or oval shape).
  • The number of outbreaks that a person experiences can vary greatly. Most people will have only a few episodes a year while, at the other extreme, others will have nearly continuous outbreaks and will never be free of ulcers for an extended period of time.

Are they contagious?

  • No, canker sores are neither contagious nor infectious.

What are "major aphthous ulcers"?

Canker sores like we have described above are formally termed "recurrent minor aphthous ulcers." They are the type of canker lesion experienced by 80% of sufferers. (Chavan 2012)

Beyond these "minor" lesions, there's another type of aphthous ulcer referred to as "major aphthae" or Sutton's ulcers. And in comparison to the minor variety they are a large deep ulceration whose healing is characteristically slower and more painful.

  • Unlike minor aphthae, the major kind forms on all types of oral tissues (both keratinized and nonkeratinized tissues). They frequently form on the lips, soft palate or fauces of the throat.
  • They can approach 1/2 inch (or more) in diameter and may cause regional or even facial swelling.
  • Their healing usually takes between 10 and 40 days. However it may take some months (even as new ulcers are forming).
  • Healing is often associated with scarring.


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